ClenbuterolClenbuterol is defined as a sympathomimetic amine; that is, its actions mimic that of adrenaline. Clenbuterol, like most B2-agonists, is used primarily for the treatment of asthma and related bronchospasm. It is a powerful bronchodilator and not only has proved useful for humans but also has widespread veterinary applications in the equine and livestock industries.
The use of clenbuterol and related B2-agonist in the livestock industry revealed a number of interesting and benefical clenbuterol side effects, namely that in high doses, B2-agonist administration produced an increase in skeletal muscle mass and a concomitant decrease in body fat. Clenbuterol is one of the few anabolic compounds that increases growth primarily through reducing muscle protein degradation.
The so-called repartitioning effects of clenbuterol are what make it so desirable for athletes, such as those involved in strength- and power-related sports and especially those involved in competitive bodybuilding.
Clenbuterol is an extremely attractive drug for athletes because of its ability to modify body structure and function in addition to the fact that it is administered orally. However, even though clenbuterol is promoted as the "safe" alternative to anabolic steroids, it does have deleterious side effects, although these do not appear to act as a deterrent to the large number of athletes abusing this drug.
Studies have been demonstrated that when B2-agonists are given in high (mg/kg) doses, extended use produces significant increases in muscle mass in mice, on the order of 10% to 25% following 10 to 20 days of administration. These doses of clenbuterol are much higher that what could be tolerated by humans. At these mg/kg doses, clenbuterol would cause muscle tremors, heat palpitations, and severe sweating in humans. The clenbuterol-induced increase in muscle protein is considered "true" muscle hyperplasia (increased cell number) and satellite cell division are not associated with the protein increases. Other B2-agonists that have proved effective in producing a muscle hypertrophic response include cimaterol, salbutamol, isoproterenol, and ractopamine, although clenbuterol ranks as one of the most potent of these compounds.
That B2-adrenoceptors mediate the anabolic effects of clenbuterol was confirmed by the actions of the non selective B1/B2-adrenoceptor antagonist, sotalol, which attenuated both the muscle growth response and the reduction in B2- adrenoceptor density. Further confirmation was obtained from the use of the antagonist ICI118551, which has a ~100-fold greater affinity for B2-adrenoceptors that for B1- adrenoceptors. ICI118551 reduced the anabolic effects of clenbuterol, and when administered alone caused muscle atrophy. These studies confirmed that B2-adrenoceptors mediate the pharmacological effects of clenbuterol and indicate that they are involved in the control of muscle growth.
The effectiveness of clenbuterol as an anabolic agent has been attributed to its ability to promote muscle protein synthesis as well as reduce muscle protein degradation. Although there has been much debate whether the increase in muscle mass is more a result of one mechanism or the other, the most recent studies attribute the anabolic properties primarily to the ability of clenbuterol to inhibit protein degradation. The exact mechanism for the protein accretion following B2-agonist administration appears to differ between species.
Anecdotal reports indicate that the muscle anabolic effects of clenbuterol in humans are much less pronounced that those observed in livestock. In fact, studies on cattle, sheep, and pigs have shown that the mechanism controlling tissue responsiveness to B2-agonist varies from species to species, and even amount different tissues within a species, primarily because of different tissues densities of the receptor subtypes. Although the muscle anabolic effects of clenbuterol appear to be much less powerful in humans compared with livestock, it is the lipolytic effects of clenbuterol that have the greatest appeal for bodybuilders.